A. Field of the Invention
This invention relates generally to wave shaping circuits and more particularly to a digital waveform shaping method and circuitry for shaping the transitions of a digital data stream.
B. Description of the Prior Art
In many communication systems, digital data streams or codes are employed as the modulating waveform for a radio transmitter. For example, in mobile, portable or paging communication systems it is common for a digital data word to be used in a manner to selectively call a given receiver. A suitable digital data stream comprises a binary sequence of equal time interval bits. Typically, a binary "1" is represented by a "high" voltage level for a bit time period, and a binary "0" is represented by a "low" voltage level for the bit time period. Generation of N successive ones results in a constant high voltage level for N successive bit periods, and N successive zeros results in a constant low voltage for a period of N bits. The resulting digital waveform, therefore, is a series of rectangular pulses with very abrupt transitions. Due to the abrupt nature of the transitions, it is difficult to confine the spectral energy of a transmission to a desired narrow bandwidth when utilizing the data bit stream to modulate the transmitter carrier. The result is the generation of undesired spurious sidebands or splatter. Such undesirable radiation increases the radio frequency bandwidth occupied by the transmission.
One approach to overcoming this problem is to filter the modulating digital data stream, but this may not prove practical in applications where amplitude distortion caused by the filtering could significantly increase the error rate at the receiver or where adequate filtering would cause unacceptable droop in the digital bits. In addition, in communication systems utilizing tone coded squelch and digital coded squelch, this approach requires switching between multiple splatter filters.
Another approach is to generate an analog ramp function and switch in the generated function for the transitions. This approach is an expensive one, and the complex circuitry is not suitable for many compact systems.
Accordingly, it is a principle object of this invention to provide digital means for digital generation of a desired data bit stream with gradual transitions thereby minimizing generation of undesired spurious sidebands when the digital data stream is used to modulate the transmitter carrier.
It is another object of the invention to provide a programmable digital means for generation of a desired digital data bit stream wherein the waveform of the bit transitions is programmable.
It is still another object of the invention to provide a digital means for a digital coded squelch communication system for digitally generating a desired digital data bit stream such that the bit transitions are made gradual prior to modulation of the carrier.
It is yet another object of the invention to provide a digital means for combined tone coded and digital coded squelch communication systems for digitally generating a desired data bit stream which eliminates the need for multiple splatter filters and which minimizes component requirements.
Briefly, according to one embodiment of the invention, there is provided digital waveform shaping apparatus for generating a digital data stream composed of a plurality of digital bits. The apparatus includes a clock for generating clock control signals coupled to a storage register which stores bits of the desired digital data stream and serially shifts the digital data stream bits to the storage register output. A detection circuit is coupled to the storage means and detects the bit values and the transitions between adjacent bits of the digital data stream at the output of the storage register. A generating circuit, responsive to the clock, is provided to generate a sequence of binary coded digital words corresponding to the magnitude of a sequence of samples of a gradual waveform. Control circuitry is coupled to the clock, the detecting circuit, and the generating circuit, to control the generating means such that digital samples of a predetermined rising portion of the gradual waveform are generated in response to a rising transition in the digital data stream. In addition, digital samples of a predetermined falling portion of the gradual waveform are generated in response to a falling transition. A digital word corresponding to a constant high level is generated after the predetermined rising portion until the beginning of the generation of a falling portion and a digital word corresponding to a constant low level is generated after the predetermined falling portion until the beginning of the generation of a rising portion. A digital to analog converter circuit is coupled to the generating circuit for converting the sequence of digital words to an analog waveform representative of the digital data stream. A filter circuit is coupled to the digital to analog converter for smoothing the analog waveform.
According to another feature of the invention, a switch input is provided for selecting one of a plurality of data words and a first memory register is provided for storing a data word. An input circuit reads the selected data word from the switch input at frequency intervals and stores the selected data word in the first memory register. A second memory register for storing a data word is also provided. A data comparator circuit compares the contents of the first memory register and the second memory register and stores the contents of the first memory register and the second memory in response to a difference in the contents of the first memory register and the second memory register. A logic circuit converts the data word in the second memory register to a corresponding desired digital data stream.
The invention as described is particularly suited for land, mobile communication systems utilizing digital coded squelch. The invention replaces conventional techniques using splatter filtering for the digital data bit stream and results in a system with reduced splatter and minimal droop and overshoot of the data signal. The invention is also compatible with systems including tone coded squelch.